Applicants claim, under 35 U.S.C. xc2xa7119, the benefit of priority of the filing date of Aug. 31, 1999 of a German patent application, copy attached, Ser. No. 199 41 318.5, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical position measuring system that is suitable for precise determination of the relative spacing of two objects movable relative to one another.
2. Discussion of Related Art
Known position measuring systems include a scanned graduation structure as a measuring graduation as well as a scanning unit, movable relative to it in the measurement direction, that has a light source, one or more graduation structures, and a detector arrangement. In the position measuring systems of interest here, the generation of the position-dependent scanning signals is based on the fundamental principles explained below. Thus, in such position measuring systems, with the aid of one or more first graduation structures, a fine fringe pattern is created, which is scanned with the aid of one of more second graduation structures to generate the scanning signals.
In the simplest case, such a position measuring system is a so-called two-grating transducer, in which a first graduation structure is illuminated by a usually collimated light source. The result is a light pattern on the downstream second graduation structure, and the graduation period of the second graduation structure matches the graduation period of the light pattern. In the case of relative motion of the two graduation structures with respect to one another, the result is a periodic modulation of the light beams that pass through the second graduation structure. The light beams that pass through the second graduation structure are detected by optoelectronic detector elements disposed downstream of the second graduation structure. Because of the functional effects of the first and second graduation structures, as they have been explained in the above example, the first graduation structure will hereinafter be called the projection graduation; conversely, the second graduation structure will be called the detection graduation.
For generating a plurality of phase-shifted scanning signals, it is often provided in such position measuring systems that the graduation periods of the detection graduation and of the projection graduation are selected as slightly different from one another. In this case, a fringe pattern whose fringes are oriented parallel to the graduation lines of the detection graduation emerges from the detection graduation. The graduation period of the emerging fringe pattern furthermore has a markedly greater graduation period than the fringe pattern on the detection graduation. For the emerging fringe patterns generated in this way, the term used below will be a so-called Vernier fringe pattern.
Alternatively, it is also possible to rotate the projection graduation and detection graduation slightly relative to one another. Then the graduation lines are not oriented parallel to one another as in the previous cases but instead have a defined small angle from one another. The result then is again an emerging fringe pattern with a markedly greater graduation period, but whose fringes are oriented perpendicular to the fringes of the detection graduation. The term used in this case is so-called moire fringe patterns.
In both cases, that is, in the creation of both Vernier fringe patterns and moirxc3xa9 fringe patterns, the actual scanning of whichever fringe pattern results is done with the aid of a further graduation structure. This graduation structure will be called a Vernier graduation for the sake of simplicity below, but it must be understood that this term does not preclude the case where a moirxc3xa9 fringe pattern is generated. The Vernier graduation must in principle have the same graduation period and graduation orientation as the Vernier or moirxc3xa9 fringe pattern generated. Only the light transmitted through the Vernier graduation then finally strikes one or more detector elements.
With regard to the Vernier graduation, it should be noted that it is also already known to embody the Vernier graduation together with a plurality of detector elements as an integral component, which is used for scanning the resultant Vernier or moirxc3xa9 fringe pattern and for generating the phase-shifted scanning signals.
While the position measuring systems discussed thus far each have included a optical collimator element, systems have also become known that work without such a optical collimator element. In this respect, see for instance the publication by R. Pettigrew, entitled xe2x80x9cAnalysis of Grating Imaging and its Application to Displacement Metrologyxe2x80x9d in SPIE, vol. 36, First European Congress on Optics Applied to Metrology (1977), pages 325-332. In such position measuring systems, a further graduation structure, hereinafter called a transmitting graduation, is additionally disposed between the light source and the projection graduation. From each transparent gap or subregion of the transmitting graduation, a beam emerges that with the aid of the projection graduation generates a periodic fringe pattern on the detection graduation. The graduation period of the transmitting graduation is selected here such that the fringe patterns emerging from the various gaps are superimposed on one another to an increased extent on the detection graduation. In this way, even light sources, such as LEDs, that are spatially far apart from one another can be used in these position measuring systems.
In summary, the above-discussed position measuring systems accordingly include at least one projection graduation and one detection graduation. Optionally, a transmitting graduation and/or a Vernier graduation can also be provided, each of which take on the functions discussed above. As a scale or measuring graduation, either the transmitting graduation, the projection graduation or the detection graduation can serve in principle in these position measuring systems.
Both the transmitting graduation and the Vernier graduation are as a rule embodied as amplitude graduation structures; conversely, the projection graduations or detection graduations can also be embodied as phase graduation structures.
An important variable in position measuring systems constructed in this way is in principle their behavior upon a change in the scanning gap. This spacing is as a rule defined by the spacing between the graduations that are movable relative to one another. Any mechanical inadequacies that may occur can then lead to more or less major fluctuations in the scanning gap during measurement operation. However, the least possible influence of such fluctuations in the scanning gap on the position-dependent output signals is to be desired.
In the aforementioned position measuring systems, there is a priori a relatively great dependency of the signal quality, detected by the detector, on the applicable scanning gap. In FIG. 9, to illustrate these problems, the resultant grating pattern contrast in a two-grating transducer is plotted in the detector plane with regard to the scanning gap. A projection graduation is used here that is embodied as a phase graduation structure, with a phase depth xcfx86=xcfx80 and a line-to-gap ratio of 1:1. It is quite clear from FIG. 9 that there are major interruptions in contrast between the maximum contrast values, and in these interruptions only low-contrast scanning signals can be detected. This in turn means that in the event of a possible undesired fluctuating scanning gap, marked sacrifices in the signal modulation result. Because of this strong dependency of the signal quality on the applicable scanning gap, stringent demands are accordingly made of the mechanical components of the applicable overall system, for instance in terms of guidance precision, etc.
From U.S. Pat. No. 5,646,730, the entire contents of which are incorporated herein by reference, a position measuring system is known in which the aforementioned scanning gap sensitivity is partly minimized. The proposed position measuring system here includes an illuminated transmission measuring graduation, which is embodied as a pure phase structure and which in accordance with the terminology introduced above functions as a projection graduation. Besides the zeroth order of diffraction, the even orders of diffraction are also suppressed by the projection graduation. A detector arrangement for detecting the displacement-dependently modulated interference pattern is placed in a region downstream of the projection graduation, in which essentially only the plus or minus 1st orders of diffraction interfere, the detector arrangement includes a plurality of individual, periodically disposed detector elements. With the terminology introduced above, this means the detection graduation, which in this case is embodied integrally with the detector arrangement or with a plurality of detector elements. In a further embodiment, it is also provided in this patent that the projection graduation be embodied as a complex phase structure, which additionally suppresses high odd orders of diffraction as well, and as a result a somewhat greater insensitivity of the scanning signals to the scanning gap can be achieved.
The spacing insensitivity sought is attained only conditionally, however, in this proposed version. For instance, a pure phase structure, despite the suppression of all the low orders of diffraction, still furnishes pairs of strong higher orders of diffraction, which can interfere with one another and, thus, lead to the undesired spacing dependency. For example, it can be demonstrated that no resultant intensity modulation can be observed immediately downstream of projection gratings embodied in this way.
Furthermore, fine structuring of the corresponding phase graduation is necessary and can accordingly be used only in systems that have coarse graduation periods; in such systems, in turn, however, the spacing dependency is fundamentally not so critical.
It is also considered a disadvantage of this position measuring system that the measuring graduation is embodied as a complex phase structure. The result is relatively major effort and expense for production, especially if a phase structure like that in FIG. 34 of U.S. Pat. No. 5,646,730, for instance, must be manufactured over a relatively long measurement length in the case of a linear measurement system. Furthermore, only a relatively small illuminated region of the projection graduation serves to generate the position-dependent scanning signals, and the result in turn is a high sensitivity of the system to soiling.
An object and advantage of the present invention is therefore to disclose an optical position measuring system in which a markedly increased tolerance to fluctuations in the scanning gap exists in the generation of the position-dependent scanning signals. Even if the scanning gap should possibly vary, an adequately constant signal modulation is needed in the event of relative motion of the graduations that are movable relative to one another. In particular, graduation structures should be as simple as possible to manufacture.
The above object and advantage are attained by an optical position measuring system that includes a light source, a measuring graduation, a scanning unit movable relative to the measuring graduation in at least one measurement direction. A projection graduation has periodic amplitude and phase structures disposed in alternation in the measurement direction. The arrangement further includes a detection graduation and a plurality of optoelectronic detector elements, wherein light from the light source interacts with the projection graduation so as to project a fringe pattern onto the detection graduation, so that via the plurality of optoelectronic detector elements, displacement-dependent output signals are detectable, and wherein the projection graduation has a structure such that in addition to even orders of diffraction and the zero order of diffraction, at least some of the (2n+1)th orders of diffraction are suppressed, where n=1, 2, 3, . . . , as a result of which essentially only the xc2x11st orders of diffraction contribute to generating the output signals.
According to the present invention, it has now been recognized that even graduation structures that are relatively simple to produce can be used in the position measuring systems discussed above to attain the stated objects and advantages, as long as certain graduation structures meet defined requirements. According to the present invention, the applicable projection graduation is embodied with alternating binary phase and amplitude structures, disposed in the measurement direction, that meet certain conditions that will be explained hereafter.
Such graduation structures are already fundamentally known from U.S. Pat. No. 4,618,214, the entire contents of which are incorporated herein by reference. However, this patent provides no suggestion whatever that such graduation structures can be used in the position measuring systems specified. Nor can specific dimensions be learned from this patent for graduation structures embodied in this way, if the graduation structures function as projection graduations in certain scanning arrangements of optical position measuring systems.
Depending on the scanning configuration, the aforementioned projection graduation can be disposed on the most various places in the scanning beam path; that is, a number of possible configurations are available within the scope of the present invention, all of them having the aforementioned advantages with respect to the low sensitivity to the scanning gap. It is understood that linear scanning arrangements can be embodied according to the present invention just as well as rotary variants.
In an advantageous embodiment of the apparatus of the invention, the respective projection graduation is designed such that along with the even orders of diffraction, at least the xc2x13rd orders of diffraction are suppressed. Already with a provision of this kind, a significant reduction in the scanning gap sensitivity can be achieved, yet without higher orders of diffraction adversely affecting the signal quality.
Further advantages, as well as details of the optical position measuring system in accordance with the invention ensue from the subsequent description of several exemplary embodiments by means of the attached drawings.